The invention relates to an FM receiver of the type including means for controlling the center frequency of an IF filter, which is relatively narrow-banded with respect to the channel bandwidth, in dependence on the desired modulation, and a signal processing unit comprising an IF unit and an LF unit for amplifying, frequency converting and demodulating the received FM signal.
Circuits of this type are disclosed in DE-A 3,146,493 and DE-A 3,438,286, EP-A 0,075,071 or FR-A 8,121,986, and originating from Jens Hansen as the inventor.
The performance capability of conventional VHF receivers is hardly able to handle present-day difficult receiving conditions. The dense occupation with stations creates a "spectral sump" which clearly shows the limits of receiver design regarding receiving sensitivity and selectivity. The dilemma of VHF reception is that the spectrum of the frequency modulated signal has no defined limits but runs out slowly and thus, inevitably, in spite of all limitation measures at the transmitter, projects into the adjacent channels. Even a medium field intensity stereo transmitter noticeably reduces the threshold sensitivity of its adjacent stations. If now, as in mobile operation, receiving conditions change constantly, increased noise, takeover of modulation, and reflection distortions are the unavoidable result.
Conventional filter technology cannot help here because the spectral lines of noise within the receiving channel can no longer be selected. Although contraction of the bandwidth to reduce multipath interferences would bring part of the noise spectrum outside of the filter, it would also bring part of the useful spectrum outside of the filter, so that the receiving sensitivity would be reduced.
In the known methods for processing FM signals in the IF domain described in the above-mentioned references, controllable, narrowband filters are employed. These narrowband filters, whose resonance position is controllable, perform at the same rhythm as the IF signal to be selected; thus they follow the momentary IF so that a selection is made at every moment precisely where a selection happens to be required. Due to the narrowbandedness of the employed filters, sensitivity and selectivity are increased considerably. The invention is based on the realization that, in order to optimize the characteristic behaviour of an FM-receiver according to the invention and during all reception conditions, any interference which is detrimental to the LF-response must be eliminated prior to it reaching the user.
During extreme adjacent channel interferences, the adjacent station may "brakes through," which leads to the filter follow-up also being taken over by the adjacent station. In particular during mobile operation, with fluctuating field intensities, breakthroughs of the adjacent station can often occur within short time spans. This leads each time to plopping noises during the LF-transmission. The object of the invention is accomplished by detecting takeover by the adjacent channel, in particular by detecting the resulting voltage leap in the low frequency LF-signal, and then cutting off the filter follow-up. The filter follow-up is discontinued for a minimum time-span, so that the cause for the noise interference is itself checked.
According to the invention, circuit which detects the adjacent channel takeover, together with other circuits which improve the processing characteristics, enhance the system because impairments to the listening enjoyment which are at first not as important become increasingly more so when all other interferences have been removed.
The control voltage for the resonant frequency of the filters is derived from the low frequency, whose level is a sufficient measure for the position of the momentary IF. However, for reasons of principle, this position indication cannot be accurate since, in order to control the filters, the IF must first pass through them and must then be demodulated. Thus, any control always takes place with a certain delay which is essentially given by the group delay of the filters. The more narrowband the filters are designed, the higher is their group delay. In simple tracking filter methods or in the known PLL or synchronous methods, this follow-up delay would have the result, in the higher low-frequency range, that the movement of the IF signal and that of the filter would go in opposite directions. Pulling of the filters in the lower LF range would be opposed by pushing in the upper LF range so that ultimately broadband transmission would be impossible.